Remote systems, such as vehicles, have been introduced that include locomotion power derived from electricity received from an energy storage device such as a battery. For example, hybrid electric vehicles include on-board chargers that use power from vehicle braking and traditional motors to charge the vehicles. Vehicles that are solely electric generally receive the electricity for charging the batteries from other sources. Battery electric vehicles (electric vehicles) are often proposed to be charged through some type of wired alternating current (AC) such as household or commercial AC supply sources. The wired charging connections require cables or other similar connectors that are physically connected to a power supply. Cables and similar connectors may sometimes be inconvenient or cumbersome and have other drawbacks. Wireless charging systems that are capable of transferring power in free space (e.g., via a wireless field) to be used to charge electric vehicles may overcome some of the deficiencies of wired charging solutions. As such, wireless charging systems and methods that efficiently and safely transfer power for charging electric vehicles are desirable.
One system of wireless power transfer is inductive power transfer (IPT). In IPT, power is transferred from a primary power device to a secondary (or “pick-up”) power device. Typically, each device includes one or more windings of electric current conveying media, such as wires, so are commonly referred to as coils.
WO 2008/140333 discloses an IPT system in which the primary and secondary devices comprise a single coil that is circular or oval in shape. Disadvantages of this arrangement include the leakage of flux from the coils and therefore poor power transfer efficiency, even where shielding is used, and a lack of tolerance away from the optimal alignment between the primary and secondary coils before a large reduction in power transfer between the coils is observed. The maneuverability of an electric vehicle may limit the ease of achieving a high degree of alignment between a pick-up coil and a primary coil, so greater tolerance is desired.
WO 2011/016736 discloses an IPT system for powering electric vehicles in which a base (usually the primary) coil, typically positioned on the ground, consists of two separate co-planar coils positioned above a core formed from a material of high magnetic permeability, such as ferrite. In this arrangement, there is no straight path through the core that passes through the coils. As such, the coils act as pole areas and lines of magnetic flux arc between them in the form of a “flux pipe” above the coils, a zone of high flux concentration. Advantageously, the arrangement results in little leakage of flux below the coils on the side of the core. However, one problem with the use of two separate coils is that the inductance of each coil can change relative to the other with different positions of the pick-up coil. When the two base coils are driven in parallel, this can result in an uneven current distribution which adversely affects the power delivery profile and magnetic field. Inefficiencies may also arise because of an increased mistuning of the system and the mutual inductance between the two base coils.
In general with IPT systems, it is desirable for the primary coil to have a low inductance. A coil with a high inductance is difficult to drive at high frequencies because large voltages are required across the coil terminals. In addition, it is difficult to form an induction coil with a low inductance that has sufficiently large physical area to be capable of charging an electric vehicle while at the same time keeping the induction coil physically thin. A physically thin coil is unobtrusive and is advantageous in wireless power transfer systems for electric vehicles where base or primary coils are positioned at ground level and the vehicle is charged by positioning a pick-up induction coil over the base coil. This is because some base coils could be positioned on top of the ground surface whereas others could be embedded in it. The thinner the base coil, the smaller the relative difference in the gap between the base and pick-up coils in these two situations. The base coil inductance may be tuned to an expected coil separation distance for optimal charging. As a result, a thin base coil means greater tolerance to different installations of a base coil relative to the ground.
Typical base coils are designed with a specific inductance and to work at a specified frequency and coil current to ensure optimal charging of batteries connected to pick-up coils designed with complimentary characteristics. Attempting to charge pick-up coils that are sub-optimally compatible with the base coil can lead to slow charging times, energy waste or overheating components. However, different types or models of vehicles may have different pick-up coils or batteries. It is therefore desirable for base chargers in a wireless charging system to be able to charge vehicles of differing characteristics with minimal loss of efficiency.